Methods are currently known in which a plurality of rotors are arranged in an axial direction of a motor shaft in an electric motor, an example of which is disclosed in Japanese Patent Application Laid-Open Publication No. 2006-14477 (JP 2006-14477 A). The conventional electric motor disclosed in JP 2006-14477 A is provided with two motor units accommodated in a single housing.
Each motor unit is comprised of a motor shaft, an inner rotor provided to the motor shaft, and a cylindrical outer stator disposed so as to surround the inner rotor. The motor shafts of the motor units are concentrically disposed and rotate in a mutually independent manner.
The motor shaft of one of the motor units extends outward from one end of the housing. The motor shaft of the other motor units extends outward from the other end of the housing.
In accordance with this type of conventional electric motor, a plurality of loads can be driven by mutually independent motor shafts.
In general, torque ripple (torque fluctuation, pulsation) can be generated in an electric motor. Torque ripple can also be generated in the prior art electric motor at certain rotational angles of the inner rotors in the two mutually independent motor shafts. It is preferable to reduce the torque ripple in order to reduce vibrations of the electric motor and have the torque be efficiently outputted from the electric motor.
It is also possible to consider providing a skew (diagonal groove) to the inner rotor and the outer stator. However, an effective magnetic flux of the inner rotor and the outer stator is reduced by an amount commensurate with the extent to which such a skew is provided. Also, the configuration of the inner rotor and the outer stator is made more complicated and the number of manufacturing steps is increased by the presence of the skew. In view of the above, there is room for further improvement.
A regenerative braking device disclosed in, e.g., Japanese Patent Application Laid-Open Publication No. 8-51701 (JP 08-51701 A), is designed to prevent battery overcharging when regenerative braking is carried out in a power apparatus provided with a drive motor.
The regenerative braking device sends regenerative current to a bypass circuit having a resistor when the battery voltage has exceeded a reference voltage (charging end-voltage) during regenerative braking. The device sets the value of the command current during regenerative braking on the basis of the battery voltage and the control state of the motor performed by the controller. When the regenerative current from the motor has exceeded the command current, current that is commensurate with the difference therebetween is sent to a regenerative resistor. As a result, battery overcharging during regenerative braking is prevented.
However, a bypass circuit resistor and a regenerative resistor for sending very large regenerative current are required in the device. These resistors must be relatively large because they convert electrical energy (regenerative current) into thermal energy and radiate the energy. Furthermore, when the amount of heat generated by the resistors is considerable, an air cooling fan or another heat dissipating device must be provided. Since this leads to a more complicated structure, there is room for improvement.
A motor generally has drive current supplied from a driver circuit. The electric current supply capability of the driver circuit must be increased when the output torque of the motor is considerable. A driver circuit having a large electric current supply capability is large in size and produces a considerable amount of heat. A heat sink must be enlarged in order to cool the driver circuit. Therefore, there is a limit to reducing the size of the power apparatus.
There are cases in which the output torque of the motor can remain low depending on the operating state of the load driven by the motor. A relatively large drive current is supplied to the motor even in such cases. There is a limit to reducing wasted electric current consumption and to improving energy savings.
Next, a self-propelled snow remover provided with a travel mechanism made capable of self-propulsion by using an electric motor such as that described above will be described. Such a snow remover is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-225308 JP 2004-225308 A).
The self-propelled snow remover described above is provided with an auger, an engine for driving the auger, a pair of left and right crawlers, and a pair of left and right electric motors for independently driving the pair of crawlers in a respective manner. The auger is driven by the engine alone. The pair of crawlers is driven by the pair of traveling electric motors alone.
When the crawlers are made to travel while removing snow using the auger, i.e., when the snow remover is made to travel and remove snow, the load on the crawlers is increased. Also, since the amount of snow removed by the auger is increased when the travel speed is increased, the crawlers are subjected to a greater load. Accordingly, snow remover is made to travel at low speed when the snow removal work is being performed. The characteristics of the traveling electric motors during snow removal should be able to provide low-speed rotation and high torque.
In a self-propelled snow remover, there are cases in which the operator desires to temporarily move (moving travel) the snow remover without having the auger perform snow removal. For example, there are cases in which the snow remover is placed in or removed from a storage location or is moved from the storage location to a nearby location for snow removal. In such cases, moving the snow remover at high speed is advantageous in terms of work efficiency. Also, since snow removal is not performed, the load on the crawlers is low. The characteristics of the traveling electric motors during moving travel should be able to provide high-speed rotation and low torque.
In this manner, the characteristics required in traveling electric motors are completely different when the self-propelled snow remover is operated for snow removal and when the snow remover is simply being moved. Electric drive motors for traveling that satisfy the characteristics of the two completely different situations are mounted on a conventional self-propelled snow remover. In other words, the traveling electric motors are capable of producing high torque. Therefore, power consumption is inevitably high in relative terms even when the snow remover is merely being moved.
In contrast, the battery for supplying power to the traveling electric motors is often a relatively low-capacity battery. The reason for this is that a large-capacity battery is not required because power can be constantly provided from the engine to the traveling electric motors via a power generator by driving the engine.
However, driving the engine in order to merely move the snow remover is a wasteful operation. It is more preferable to be able to drive the traveling electric motor by supplying power only from a low-capacity battery when the snow remover is merely being moved.